198 research outputs found
Evolution and instabilities of disks harboring super massive black holes
The bar formation is still an open problem in modern astrophysics. In this
paper we present numerical simulation performed with the aim of analyzing the
growth of the bar instability inside stellar-gaseous disks, where the star
formation is triggered, and a central black hole is present. The aim of this
paper is to point out the impact of such a central massive black hole on the
growth of the bar. We use N-body-SPH simulations of the same isolated
disk-to-halo mass systems harboring black holes with different initial masses
and different energy feedback on the surrounding gas. We compare the results of
these simulations with the one of the same disk without black hole in its
center. We make the same comparison (disk with and without black hole) for a
stellar disk in a fully cosmological scenario. A stellar bar, lasting 10 Gyrs,
is present in all our simulations. The central black hole mass has in general a
mild effect on the ellipticity of the bar but it is never able to destroy it.
The black holes grow in different way according their initial mass and their
feedback efficiency, the final values of the velocity dispersions and of the
black hole masses are near to the phenomenological constraints.Comment: 10 pages, 8 figures, accepted for pubblication in "Astrophysics and
Space Science
New Signatures of the Milky Way Formation in the Local Halo and Inner Halo Streamers in the Era of Gaia
We explore the vicinity of the Milky Way through the use of
spectro-photometric data from the Sloan Digital Sky Survey and high-quality
proper motions derived from multi-epoch positions extracted from the Guide Star
Catalogue II database. In order to identify and characterise streams as relics
of the Milky Way formation, we start with classifying, select, and study
subdwarfs with up to kpc away from the Sun as tracers
of the local halo system. Then, through phase-space analysis, we find
statistical evidence of five discrete kinematic overdensities among of the
fastest-moving stars, and compare them to high-resolution N-body simulations of
the interaction between a Milky-Way like galaxy and orbiting dwarf galaxies
with four representative cases of merging histories. The observed overdensities
can be interpreted as fossil substructures consisting of streamers torn from
their progenitors, such progenitors appear to be satellites on prograde and
retrograde orbits on different inclinations. In particular, of the five
detected overdensities, two appear to be associated, yelding twenty-one
additional main-sequence members, with the stream of Helmi et al. (1999) that
our analysis confirms on a high inclination prograde orbit. The three newly
identified kinematic groups could be associated with the retrograde streams
detected by Dinescu (2002) and Kepley et al. (2007), whatever their origin, the
progenitor(s) would be on retrograde orbit(s) and inclination(s) within the
range . Finally, we use our simulations to
investigate the impact of observational errors and compare the current picture
to the promising prospect of highly improved data expected from the Gaia
mission.Comment: 16 pages, 10 figures, 6 Tables. Accepted for publication in The
Astronomical Journa
Gaseous Flows in Galaxies
The gas component plays a major role in the dynamics of spiral galaxies,
because of its dissipative character, and its ability to exchange angular
momentum with stars in the disk. Due to its small velocity dispersion, it
triggers gravitational instabilities, and the corresponding non-axisymmetric
patterns produce gravity torques, which mediate these angular momentum
exchanges. When a srong bar pattern develops with the same pattern speed all
over the disk, only gas inside corotation can flow towards the center. But
strong bars are not long lived in presence of gas, and multiple-speed spiral
patterns can develop between bar phases, and help the galaxy to accrete
external gas flowing from cosmic filaments. The gas is then intermittently
driven to the galaxy center, to form nuclear starbursts and fuel an active
nucleus. The various time-scales of these gaseous flows are described.Comment: 10 pages, 6 figures, review paper in the Proceedings of the IAU
Symposium 245, "Formation and Evolution of Galaxy Bulges", held at Oxford,
U.K., July 2007, Eds. M. Bureau, E. Athanassoula, B. Barbu
Evolution of stellar-gaseous disks in cosmological haloes
We explore the growth and the evolution of the bar instability in stellar-gaseous disks embedded in a suitable dark matter halo evolving in a fully consistent cosmological framework. The aim of this paper is to point out the impact of different gas fractions on the bar formation, inside disks of different disk-to-halo mass ratio, and the role of the cosmological framework. We perform cosmological simulations with the same disk-to-halo mass ratios as in a previous work where the gas was not taken into account. We compare results of the new simulations with the previous ones to investigate the effect of the gas by analysing the morphology of the stellar and gaseous components, the stellar bar strength and the behaviour of its pattern speed. In our cosmological simulations, inside dark-matter dominated disks, a stellar bar, lasting 10 Gyr, is still living at z=0 even if the gaseous fraction exceeds half of the disk mass. However, in the most massive disks we find a threshold value (0.2) of the gas fraction able to destroy the bar. The stellar bar strength is enhanced by the gas and in the more massive disks higher gas fractions increase the bar pattern speed
Star formation and bar instability in cosmological haloes
This is the third of a series of papers presenting the first attempt to
analyze the growth of the bar instability in a consistent cosmological
scenario. In the previous two articles we explored the role of the cosmology on
stellar disks, and the impact of the gaseous component on a disk embedded in a
cosmological dark matter halo. The aim of this paper is to point out the impact
of the star formation on the bar instability inside disks having different gas
fractions. We perform cosmological simulations of the same disk-to-halo mass
systems as in the previous works where the star formation was not triggered. We
compare the results of the new simulations with the previous ones to
investigate the effect of the star formation by analysing the morphology of the
stellar components, the bar strength, the behaviour of the pattern speed. We
follow the gas and the central mass concentration during the evolution and
their impact on the bar strength. In all our cosmological simulations a stellar
bar, lasting 10 Gyr, is still living at z=0. The central mass concentration of
gas and of the new stars has a mild action on the ellipticity of the bar but is
not able to destroy it; at z=0 the stellar bar strength is enhanced by the star
formation. The bar pattern speed is decreasing with the disk evolution.Comment: 10 pages, 21 figures, A&A accepte
The Asymmetric Thick Disk: A Star Count and Kinematic Analysis. II The Kinematics
We report a kinematic signature associated with the observed asymmetry in the
distribution of thick disk/inner halo stars interior to the Solar circle
described in Paper I. In that paper we found a statistically significant excess
(20% to 25 %) of stars in quadrant I (l ~ 20 deg to 55 deg) both above and
below the plane (b ~ +/- 25 deg to +/- 45 deg) compared to the complementary
region in quadrant IV. We have measured Doppler velocities for 741 stars,
selected according to the same magnitude and color criteria, in the direction
of the asymmetry and in the corresponding fields in quadrant IV. We have also
determined spectral types and metallicities measured from the same spectra. We
not only find an asymmetric distribution in the V_LSR velocities for the stars
in the two regions, but the angular rate of rotation, w, for the stars in
quadrant I reveals a slower effective rotation rate compared to the
corresponding quadrant IV stars. We use our [Fe/H] measurements to separate the
stars into the three primary population groups, halo, thick disk, and disk, and
conclude that it is primarily the thick disk stars that show the slower
rotation in quadrant I. A solution for the radial, tangential and vertical
components of the V_LSR velocities, reveals a significant lag of ~ 80 to 90
km/s in the direction of Galactic rotation for the thick disk stars in quadrant
I, while in quadrant IV, the same population has only a ~ 20 km/s lag. The
results reported here support a rotational lag among the thick disk stars due
to a gravitational interaction with the bar as the most likely explanation for
the asymmetry in both the star counts and the kinematics. The affected thick
disk stars, however, may be associated with the recently discovered Canis Major
debris stream or a similar merger event (abridged).Comment: Accepted for publication in the Astronomical Journa
The radial metallicity gradients in the Milky Way thick disk as fossil signatures of a primordial chemical distribution
In this letter we examine the evolution of the radial metallicity gradient
induced by secular processes, in the disk of an -body Milky Way-like galaxy.
We assign a [Fe/H] value to each particle of the simulation according to an
initial, cosmologically motivated, radial chemical distribution and let the
disk dynamically evolve for 6 Gyr. This direct approach allows us to take into
account only the effects of dynamical evolution and to gauge how and to what
extent they affect the initial chemical conditions. The initial [Fe/H]
distribution increases with R in the inner disk up to R ~ 10 kpc and decreases
for larger R. We find that the initial chemical profile does not undergo major
transformations after 6 Gyr of dynamical evolution. The final radial chemical
gradients predicted by the model in the solar neighborhood are positive and of
the same order of those recently observed in the Milky Way thick disk.
We conclude that: 1) the spatial chemical imprint at the time of disk
formation is not washed out by secular dynamical processes, and 2) the observed
radial gradient may be the dynamical relic of a thick disk originated from a
stellar population showing a positive chemical radial gradient in the inner
regions.Comment: 10 pages, 5 figures, Accepted for publication on Astrophysical
Journal Letter
Self-Consistent Evolution of Ring Galaxies
Ring galaxies are commonly known as objects where a burst of star formation
was triggered by a close encounter with an intruder, maybe a satellite galaxy.
BVRI CCD observations of five ring galaxies have been performed. Here we
present the results of a self-consistent approach to reproduce their observed
morphology and spectral energy distribution using updated --body simulations
and evolutionary population synthesis models extending from UV to far--IR
wavelengths. Some suggestions about the evolutionary properties of these
starburst galaxies are then derived.Comment: MNRAS accepted; latex file, 20 pages, 3 tables and 11 figures
available upon reques
Assembly of the outer Galactic stellar halo in the hierarchical model
We provide a set of numerical N-body simulations for studying the formation
of the outer Milky Ways's stellar halo through accretion events. After
simulating minor mergers of prograde and retrograde orbiting satellite halo
with a Dark Matter main halo, we analyze the signal left by satellite stars in
the rotation velocity distribution. The aim is to explore the orbital
conditions where a retrograde signal in the outer part of the halo can be
obtained, in order to give a possible explanation of the observed rotational
properties of the Milky Way stellar halo. Our results show that, for satellites
more massive than of the main halo, the dynamical friction has a
fundamental role in assembling the final velocity distributions resulting from
different orbits and that retrograde satellites moving on low inclination
orbits deposit more stars in the outer halo regions end therefore can produce
the counter-rotating behavior observed in the outer Milky Way halo.Comment: 5 pages, 3 figures, ApJL, accepte
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